Preheat oven to 400 degrees, with racks in upper and lower thirds. Place beef on a large rimmed baking sheet; season generously with salt and pepper. Roast on upper rack for 45 minutes; move to lower rack.
Remove 3 to 4 tablespoons of fat from beef and place on a second rimmed baking sheet with potatoes, garlic, and rosemary. Season with salt and pepper and toss to combine; place on upper rack.
Roast beef until an instant-read thermometer inserted in the thickest part registers 110 degrees, 45 to 60 minutes more. Remove beef from oven and tent loosely with aluminum foil; let rest until internal temperature registers 130 degrees, about 30 minutes.
Continue to roast potatoes, tossing occasionally, until golden, 25 to 30 minutes more. Slice beef and serve with potatoes.